This invention relates to the preparation of silver halide emulsions. In a particular aspect it relates to preparation of such emulsions containing iridium by using as the source of iridium a nitric acid solution of an iridium salt. In another aspect, it relates to emulsions prepared by this process.

Since the middle l940's, iridium has been described as an addendum to silver halide emulsions to modify their photographic properties. Smith and Trivelli, in U.S. Patents 2,448,060, 2,566,245, and 2,566,263 describe the use of salts of Group VIII metals, including iridium, to increase the sensitivity of a silver halide emulsion. The ′060 and ′245 patents describe the use of tetravalent metal salts while the ′263 patent describes the use of the trivalent metal salt.

Wark U.S. Patent 2,7l7,833; Berriman U.S. Patent 3,367,778; Bacon U.S. Patent 3,53l,29l and Bigelow U.S. Patent 3,6l5,579 describe the use of iridium salts in the preparation of light-developable silver halide emulsions and direct positive silver halide emulsions.

Gilman et al U.S. Patent 3,979,2l3 describes the use of iridium as a metal dopant in internal image silver halide emulsions.

Research Disclosure, May l986, Item 26253, describes the use of iridium salts in combination with sulphur sensitizers in the preparation of silver halide emulsions. Research Disclosure is published by Kenneth Mason Publications, Ltd., The Old Harbourmaster's, 8 North Street, Emsworth, Hampshire P0l0 7DD, England.

Defensive Publication T962,004, published September 6, l977, describes the use of iridium salts with water soluble purine bases or salts.

Ihama et al U.S. Patent 4,693,965 issued September l5, l987, describes ripening of a silver halide emulsion in the presence of an iridium salt and a photographic spectral sensitizing dye.

A review of the various ways in which iridium has been used in the preparation of silver halide emulsions is contained in B. H. Carroll "Iridium Sensitization: A Review", Photographic Science and Engineering, 24:265-267, l980.

The literature to date, when discussing iridium salts, equates salts of iridium (III) and iridium (IV), and describes their incorporation into the silver halide emulsion from aqueous solutions.

For preparation of small batches of emulsion, that do not require a long period of time, the solution stability of the iridium salt is not of concern. However, in a manufacturing environment, it often is necessary to keep a solution for several hours during the preparation of an emulsion. It then is desirable that the solution be as stable as possible, so as to minimize variations in the composition of the solution from the start of the preparation to the end of the preparation which might result in differences in performance of the emulsion made at the beginning of the preparation and those made at the end.

A problem to be solved by this invention is to provide processes of preparing silver halide emulsions containing iridium, in which the iridium salt solution is relatively stable.

This problem is solved by using as the iridium salt an iridium (IV) salt in an aqueous nitric acid solution.

One embodiment of our invention is a process for preparing silver halide emulsions containing iridium by treating the emulsion, during or after precipitation of the silver halide grains, with an aqueous solution of an iridium salt, characterized in that the iridium salt solution is a nitric acid solution of an iridium (IV) salt. Another embodiment of this invention is an emulsion prepared by this process.

As the iridium salt there can be used any of the iridium (IV) salts and complexes available in the art to modify the properties of silver halide emulsions. Iridium halides, alkali metal iridium halides, alkaline earth metal iridium halides, ammonium iridium halides and alkyl-and aryl-ammonium iridium halides are suitable. In a preferred embodiment, the iridium salt is dipotassium hexachloroiridate (IV).

The iridium salt can be present in the solution in a concentration of 0.0l to 20 grams/liter, depending upon the purpose and manner of addition to the silver halide emulsion.

Preferably, the solution is from 0.0l to l0.0 Normal in nitric acid. Especially preferred are 0.l to 5.0 Normal nitric acid solutions. Most preferred are l.0 to 4.0 Normal nitric acid solutions.

The solution should be free of species which tend to destabilize the iridium (IV) salt. Such species are reducing agents, such as halides, and weak oxidizing agents, such as potassium nitrate.

The iridium (IV) salts, and their nitric acid solutions can be prepared from commercially available trivalent iridium salts by oxidation with nitric acid. The resulting nitric acid solution can be used as is, or the iridium (IV) salt can be separated by precipitation and redissolved in an aqueous nitric acid solution. Methods for purification of iridium salts are described in MacGregor U.S. Patents 3,960,549 and 3,979,207; and a typical preparation of a solution useful in the practice of this invention is shown in Example l, infra.

The iridium salt solutions employed in this invention car be used to modify the properties of silver halide emulsions in the ways and for the purposes that iridium salt solutions have been used in the prior art. Thus, the iridium salt solution car be added to the emulsion during precipitation of the emulsion to include iridium in the silver halide grain, or it can be added after precipitation of the emulsion to add iridium to the surface of the grain. Iridium can be used to increase the sensitivity of the emulsion, to improve or modify its development characteristics, to reduce reciprocity failure, to impart internal sensitivity, to stabilize it against fog, and/or to decrease its sensitivity to physical manipulation.

Depending on the purpose for which the iridium salt is added to the emulsion, it can be present in amounts from l0⁻⁸ to l0^-l mols iridium per mol silver.

In addition, the emulsion car be chemically and/or spectrally sensitized prior to, at the same time as, or subsequent to addition of iridium.

In a preferred embodiment, the iridium salt solution is employed with negative-working silver bromide and silver chloride emulsions, or combinations thereof with each other and/or with silver iodide. Specific preferred silver halide emulsions include silver chloride, silver bromide, silver chlorobromide and silver bromoiodide emulsions.

In a particularly preferred embodiment, the iridium salt is used to increase the sensitivity of the silver halide grains by adding iridium to the emulsion subsequent to precipitation and/or to decrease reciprocity failure by incorporation during precipitation. Especially preferred is use of iridium in combination with a chemical sensitizer, such as sulfur, noble metals, e.g. gold, and combinations thereof and/or a spectral sensitizer such as cyanine or merocyanine dye.

The emulsions prepared in accordance with this invention can be used in the ways that such emulsions have previously been used in the art. Representative uses include color photographic elements, including color negative and color reversal films and papers, and in black-and-white elements such as X-ray and lithographic films and papers. Details of such use are described in Research Disclosure December l978, Item l7643, pp 2l-3l; Ressearch Disclosure August l979, Item l8434, pp 433-44l; and Research Disclosure November l979, Item l87l6, pp 647-65l.

The following examples further illustrate this invention.

Tripotassium hexachloroiridate, K₃IrCl₆, (l50 gm, 0.24 moles) was dissolved in distilled water (2 l). In succession, concentrated hydrochloric acid (300 ml) and nitric acid (200 ml) were added to the solution. The resulting solution was heated slowly to 90°C and then allowed to stand until it had cooled to room temperature. Potassium chloride (298 gm) was added to the solution, followed by the addition of sufficient distilled water to bring the total volume to 3.5 l. Stirring was continued until all potassium chloride had dissolved. After chilling to l0°C during which time the product precipitated from solution, the suspension was filtered through a coarse glass filter by suction filtration. The product was washed on the funnel, first by a chilled (l0°C) solution of potassium chloride (3l gm in 475 ml water) and then with ethanol (500 ml). The product was dried at l05°C. A yield of ll3.5 gm was obtained, which was 82% of the theoretically possible yield.

The Table l, below, contains a summary of the solution stabilities of the title salt. Comparisons were made in distilled water, aqueous nitric acid of varying normality, aqueous potassium nitrate, and aqueous solutions of mixtures of sodium chloride and sodium bromide. The table illustrates that the aqueous nitric acid solution is by far the most stable. Stability is expressed as the time required for 5% of the iridium (IV) salt to decompose to other species as measured by a 5% decrease in the absorption spectra attributed to the iridium (IV) salt.
Samples l, 3 and 4 show that nitric acid solutions of the iridium salt have significantly increase stability compared to a control from which the nitric acid was omitted or a comparison, sample 5, with potassium nitrate. Comparison samples 6 and 7 illustrate the effect of destabilizing species such as potassium nitrate and sodium halides.